Abstract
Quantum theory was initiated by Max Planck’s discovery, in 1900, of his black body radiation law, which revealed, or in any event compelled Planck to postulate, that radiation, such as light, previously considered a continuous phenomenon in all circumstances, could also exhibit features of discontinuity or discreteness in certain circumstances. The limit at which this discontinuity appears is defined by the frequency of the radiation and a universal constant of a very small magnitude, h, Planck’s constant, which Planck termed “the quantum of action” and which turned out to be one of the most fundamental constants of physics. The indivisible (energy) quantum of radiation in each case is the product of h and the frequency, E = hv. The role of h may be seen as analogous to the role of c (the constancy of the speed of light in a vacuum in its independence of the speed of the source) in special relativity, in terms of both the necessity of a departure from classical physics and of introducing the first principles of a new theory. Other earlier developments, sometimes referred to as the “old quantum theory,” made apparent yet further complexities of quantum phenomena and posed new questions concerning them.
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Notes
- 1.
Most key founding papers on quantum mechanics are assembled in (van der Waerden 1968), and will be cited from this volume. Throughout this study, by “quantum mechanics” I mean the standard version of quantum mechanics, covered by Heisenberg’s or Schrödinger’s formalism, or other, more or less mathematically equivalent, versions of the formalism, such as those of Paul Dirac or John von Neumann. The term will not refer to alternative accounts of the experimental data in question, such as those offered by Bohmian mechanics, for example (and there are several versions of the latter). “Quantum theory” will refer more generally to theoretical thinking concerning quantum phenomena. This denomination, thus, includes the old quantum theory; alternatives to standard quantum mechanics, such as those just mentioned; and higher level quantum theories, such as quantum electrodynamics and other quantum field theories. “Quantum phenomena” will refer to those observable phenomena in considering which the role of Planck’s constant h cannot be neglected, as it can be in the case of the phenomena considered by classical physics. “Quantum physics” will refer to the overall assembly of experimental and theoretical accounts of these phenomena. “Classical mechanics” (or “Newtonian mechanics”), “classical theory,” and “classical physics” will be used along respectively parallel lines.
- 2.
“The Quantum of Action and the Description of Nature,” in Philosophical Writings of Niels Bohr, 3 vols (Bohr 1987, vol. 1, p. 92). This collection will hereafter be referred to as PWNB, followed by a volume number. A supplementary volume of Bohr’s essays was published as Niels Bohr, The Philosophical Writings of Niels Bohr, Volume IV: Causality and Complementarity, Supplementary Papers, J. Faye and H. Folse, ed. (Bohr 1998), and it will be referred as PWNB 4. These four volumes contain most of Bohr’s works on quantum mechanics and complementarity to be cited here. The key articles are listed separately in the bibliography with the original publication date, to be given in the text as well: thus, the article just cited will be referred to as “Bohr 1929a, PWNB 1,” followed by page numbers.
- 3.
On Einstein’s work on this subject, see (Pais 1982, pp. 402–14) and (Pais 1991, pp. 191–192).
- 4.
An uncertainty relation applies to the coordinate and the momentum in a given direction. For a quantum object in three-dimensional space, each quantity will have three components, defined by the chosen coordinate system.
- 5.
It is usually assumed that a cause precedes its effect, or is simultaneous with it. Relativity further restricts the application of the concept of causality by the assumption that causal influences cannot travel faster than the speed of light in a vacuum, c, the assumption known as the principle of locality. Sometimes, the term “causality” is used in this sense. When speaking of the lack of causality in quantum mechanics, I only mean the inapplicability of the concept of causality found in classical physics and not any form of incompatibility with relativity.
- 6.
The terms “empiricism” and “positivism,” and the philosophical views with which they are associated have a long history and encompass a broad spectrum of positions. They do not, in general, have the same meaning, although their meanings are related and sometimes converge, as in the case of logical positivism, also known as logical empiricism. Roughly, empiricism, especially as initiated by the philosophy of John Locke, is a theory of knowledge based on the claim that knowledge derives most essentially from sensory experience. Positivism, especially as a form of philosophy of science, commonly adds logical and mathematical treatment of the empirical data as the second essential source of all real knowledge. See, for example, the corresponding entries in Stanford Encyclopedia of Philosophy (http://plato.stanford.edu/) for further discussion. Whatever the further complexities of empiricism and positivism, or of Mach’s philosophy, the most essential point of difference between Einstein’s realist thinking or, in this respect, Bohr’s nonrealistic thinking and positivist or empiricist views is defined by the fundamental role, for both Einstein and Bohr, of concepts and hence of human thought in scientific knowledge. For Einstein, the creation or use of concepts always stands in the service of scientific realism, but it need not do so, and it does not for Bohr. Einstein’s view is close to Kant and his followers, especially Hegel, who in part advanced their views of the role of concepts in theoretical thinking against empiricism, such as that of Locke and Hume. For Kant and Hegel, thought and concepts shape and even determine what is, or appears to be, observed by senses.
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© 2013 Arkady Plotnitsky
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Plotnitsky, A. (2013). 1900–1962: From Planck to Bohr. In: Niels Bohr and Complementarity. SpringerBriefs in Physics. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-4517-3_1
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